A well-established method of thiocyanating aminobenzenes, which typically is carried out as a first step in the synthesis of 2-aminobenzothiazoles, comprises reacting the aminobenzene compound with an alkali metal- or ammoniumthiocyanate in a neutral solvent under bromine- or chlorine-catalyzed oxidizing conditions. The thiocyanation reaction is commonly understood to proceed via oxidation of thiocyanate ion, SC.dbd.N.sup.-, to form a reactive thiocyanogen entity, (N.dbd.C--S).sub.2, the lability of which essentially requires that it be generated in situ in the presence of the aniline compound, see Organic Reactions, Vol. III, Adams, R. ed., John Wiley & Sons, Inc., New York (1946), pp. 251-253. However, the halogen and cyanide-contaminated effluent of such chemical thiocyanation processes poses a substantial problem of waste treatment in face of increasingly stringent environmental regulations.
Over the past fifty years or longer, workers in the art have investigated an electrochemical process for thiocyanating aminobenzenes and other organic compounds, see Adams, ed., id. at 253, 257. Such an anodic oxidation process, which dispenses with the requirement of a halogen reagent, potentially meets the need for a more environmentally compatible thiocyanation process.
The approach heretofore in the art, however, to applicant's knowledge, is of limited practicality for industrial purposes. For example, workers in the art have carried out a laboratory electrochemical thiocyanation reaction at temperatures well below the ambient (i.e. down to about -5.degree. to -8.degree. C. and even lower) in order to stabilize the thiocyanogen radical, see, e.g., N. N. Mel'nikov and E. M. Cherkasova, J.Gen. Chem. (U.S.S.R.) 14, 113-115 (1944); but at more favorable reaction temperatures (e.g., 5.degree.-15.degree. C.) under the prior art conditions, hydrogen cyanide has proved to be a significant by-product. Furthermore, in the prior, "low temperature" processes, an ethanol solvent has been employed which is diluted with acid or water to preserve conductivity, to an extent, however, which can affect reactant solubility. Other reported work involving the use of toxic and/or expensive solvents, such as acetonitrile (see C.R. Acad. Sc. Paris, t.266, Mar. 18, 1968, Series C-883), has limited application on a commercial scale.
Moreover, acid corrosion of the cathode in a scaled-up single cell electrochemical thiocyanation process, is a significant operating difficulty which has still to be addressed.
It has therefore been an objective in the art to devise an industrially feasible electrochemical process for thiocyanating aminobenzenes.
It has been a particular objective to achieve a process in which cathodic corrosion is reduced or prevented.
Such a process would provide substantial commercial advantages, for example, from the standpoint of reducing toxic effluent, in a process for thiocyanating aminobenzene compounds, and in an overall process for preparing 2-aminobenzothiazole compounds.